Process for manufacturing a steel tube for air bags

ABSTRACT

A seamless steel tube for air bag use is formed from a steel comprising, in mass percent, C: 0.04-0.20%, Si: 0.10-0.50%, Mn: 0.10-1.00%, P: at most 0.025%, S: at most 0.005%, Al: at most 0.10%, Cr: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, a remainder of Fe and unavoidable impurities. The tube is cold drawn at least one time with a working ratio so that reduction in area is greater than 40% to obtain predetermined dimensions. The tube is then quench hardened by heating to at least the Ac 3  point at a temperature rate increase of at least 50° C. per second followed by cooling at a cooling rate of at least 50° C. per second at least in a temperature range of 850-500° C. Tempering is done at a temperature of at most the Ac 1  point.

TECHNICAL FIELD

This invention relates to a process for inexpensively manufacturing aseamless steel tube which is suitable as a steel tube for air bags (airbag systems) and of which are required a high strength as expressed by atensile strength of at least 900 MPa and a high level of toughness asexpressed by a value of vTrs100 (the lowest Charpy fracture appearancetransition temperature at which the percent ductile fracture is 100%) of−60° C. or below.

BACKGROUND ART

In recent years, the automotive industry has actively promoted theintroduction of safety equipment. One example of such equipment whichhas been developed is an air bag system, which has been installed inmany automobiles. At the time of a collision, an air bag system inflatesan air bag with a gas or the like between a passenger and the steeringwheel, the instrument panel, or the like before the passenger impactsthese objects and reduces injuries of the passenger by absorbing thekinetic energy thereof. Air bag systems were initially of a type whichused explosive chemicals, but in recent years, a type which uses ahigh-pressure filling gas has been developed and is being increasinglywidely used.

In air bag systems which use a high-pressure filling gas, an inflatinggas such as an inert gas (such as argon) which is blown into an air bagat the time of a collision is always maintained at a high pressureinside an accumulator connected to the air bag, and at the time of acollision, the gas is blown all at once from the accumulator into theair bag in order to inflate the air bag. An accumulator is typicallymanufactured by welding a lid to both ends of a steel tube which hasbeen cut to a suitable length and if necessary subjected to diameterreduction.

Accordingly, a stress at a high strain rate is applied to a steel tubeused for an accumulator of an air bag system (referred to below as anair bag accumulator or simply as an accumulator) in an extremely shortlength of time. Therefore, unlike structures such as conventionalpressure cylinders or line pipes, this type of steel tube requires highdimensional accuracy, workability, and weldability as well as a highstrength and excellent bursting resistance.

Recently, there are increasing demands for decreases in the weight ofautomobiles. From this standpoint, there is also a desire to decreasethe wall thickness and the weight of steel tubes for air bags formounting on automobiles. In order to guarantee a high bursting pressureeven with a decreased wall thickness, accumulators are now manufacturedfrom high-strength seamless steel tubes having a tensile strength of atleast 900 MPa or even at least 1000 MPa. Taking an accumulatormanufactured from a seamless steel tube having an outer diameter of 60mm and a wall thickness of 3.55 mm as an example, if its tensilestrength is 800 MPa, its bursting pressure is at most around 100 MPa,but if its tensile strength is 1000 MPa, its bursting pressure increasesto 130 MPa. At the same time, when the outer diameter of an air bagaccumulator and the required bursting pressure are constant, it ispossible to decrease the wall thickness by around 20%.

An accumulator also needs to have excellent low-temperature toughness sothat even in cold regions, the accumulator does not undergo brittlefracture at the time of a collision which can lead to secondaryaccidents.

For this reason, a seamless steel tube for an accumulator has beenimparted a high strength and a high toughness by carrying out quenchhardening and tempering thereon. Specifically, it is desired that anaccumulator have low-temperature toughness such that fracture in aCharpy impact test at −60° C. is ductile (namely, vTrs100 is −60° C. orbelow) and preferably such that fracture in a Charpy impact test at −80°C. is ductile (vTrs100 is −80° C. or below).

Concerning a seamless steel tube for air bag systems having a highstrength and high toughness, Patent Document 1, for example, proposes aprocess for manufacturing a seamless steel tube for air bags comprisingforming a seamless steel tube by hot working using a steel materialhaving a chemical composition in a prescribed range, cold drawing theseamless steel tube so as to give predetermined dimensions, heating thesteel tube to a temperature in the range of at least the Ac₃ point to atmost 1050° C. followed by quenching, and then tempering it at atemperature in the range of at least of 450° C. to at most the Ac₁point.

It is purported that this process provides a seamless steel tube whichhas excellent workability and weldability at the time of manufacture ofan air bag inflator, which has a tensile strength of at least 900 MPawhen used as an inflator, and which has high toughness such that itexhibits ductility in a dropping test performed at −60° C. on a steeltube cut in half. However, the fact that it exhibits ductility in adropping test at −60° C. does not necessarily mean that it is ductile ina bursting test at −60° C.

Patent Document 2 proposes a process for manufacturing a steel tube forair bag systems having a tensile strength exceeding 1000 MPa by carryingout quench hardening by high-frequency induction heating to achievegrain refinement by rapid heating. When using a seamless steel tube as amother tube, the seamless steel tube is prepared by hot tube formingusing a steel material having a chemical composition in a prescribedrange, and the seamless steel tube is subjected to cold drawing toobtain a steel tube having predetermined dimensions. After the steeltube is heated, it is quenched and then tempered at a temperature of atmost the Ac₁ transformation point. By carrying out tempering afterquench hardening, the steel tube is given a desirable high toughness soas to exhibit ductility in a bursting test even at −80° C. or below.

However, in the processes disclosed in Patent Documents 1 and 2, asspecifically disclosed therein, in order to obtain a steel tube having atensile strength of at least 1000 MPa and a high toughness, it wasnecessary to contain a large amount of expensive alloying metals such asCr and Mo. In Patent Document 1, the (Cr+Mo) content is from 1.0 to 2.5mass %, and in Patent Document 2, a steel material is employed for whichin many cases the (Cr+Mo) content is 0.92 mass %. If large amounts of Crand Mo are contained, in addition to a high material cost particularlydue to expensive Mo, after forming a seamless steel tube in a hot state,the resulting steel tube tends to have a high strength which makes thesubsequent cold drawing difficult. Therefore, softening treatmentbecomes necessary before cold drawing, thereby making the manufacturingprocess complicated and manufacturing costs high.

Patent Document 3, which utilizes a steel in which the (Cr+Mo) contentis 1.0-1.18 mass %, has the same problems as Patent Documents 1 and 2.

Patent Document 4 discloses a steel composition for a seamless steeltube having excellent bursting resistance and which contains Cr, Mo, Cu,and Ni. However, its properties are evaluated with respect to a seamlesssteel tube in which the (Cr+Mo) content is at least 0.76 mass %, and thetensile strength of that tube is at most 947 MPa.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: JP 2004-76034 A1-   Patent Document 2: WO 2004/104255 A1-   Patent Document 3: US 2005/0076975 A1-   Patent Document 4: WO 2002/079526 A1

SUMMARY OF THE INVENTION

In a conventional steel tube for air bags, in order to provide it with ahigh strength and a high toughness, strengthening was achieved by addingCr and Mo. However, that technique not only increases the alloy cost butalso makes it difficult to carry out cold drawing after tube forming.Therefore, when there is a large difference between the size of aseamless steel tube used as a mother tube and the size of a steel tubefor air bags as a final product, it becomes necessary to repeat colddrawing multiple times in a cold drawing step. In this case, the steeltube is finished to a product with desired dimensions while carrying outsoftening between successive times of cold drawing, so the overallmanufacturing costs increase.

An object of the present invention is to provide a process formanufacturing a steel tube for air bags having a high strength and hightoughness by less expensive means than the prior art techniques andwhich is less expensive than conventional products by simplifying adrawing step or decreasing the alloy cost.

From another standpoint, an object of the present invention is toprovide a process for manufacturing a steel tube for air bags having awall thickness and diameter which are the same as or smaller than thoseof conventional products using a starting material and a manufacturingprocess with lower costs than in the past.

The present inventors noted that as a result of relying on strengtheningby Cr and Mo in a conventional high-strength steel tube for air bags,the strength after the completion of hot tube forming becomes high,thereby leading to a decrease in productivity during cold drawing, andthe alloy cost increases. Therefore, they investigated an alloycomposition and a manufacturing process which suppress the use of thesealloy elements as much as possible and which can guarantee a highstrength as expressed by a tensile strength of at least 900 MPa andexcellent low-temperature toughness as expressed by vTrs100 of −60° C.or below.

As a result, they obtained the following knowledge and completed thepresent invention.

(a) In the manufacture of a steel tube for air bags by carrying out colddrawing followed by quench hardening and tempering, if the heatingconditions and cooling conditions at the time of quench hardening areappropriately set, it is possible to guarantee a high strength andlow-temperature toughness even if the steel tube does not contain alarge amount of Cr and Mo. It is particularly effective for the steel tocontain Cu and Ni in place of Cr and Mo.

(b) A steel having a reduced content of Cr and Mo and in placecontaining Cu and Ni easily undergoes cold drawing after hot tubeforming. As a result, it is possible to increase the working ratio(reduction in area) in one time of cold drawing operation in a colddrawing step, thereby simplifying the cold drawing step.

The present invention is a process for manufacturing a steel tube forair bags characterized by including a tube forming step in which aseamless steel tube is produced by hot tube forming from a steelcomprising, in mass %, C: 0.04-0.20%, Si: 0.10-0.50%, Mn: 0.10-1.00%, P:at most 0.025%, S: at most 0.005%, Al: at most 0.10%, Cr: 0.01-0.50%,Cu: 0.01-0.50%, Ni: 0.01-0.50%, and a remainder of Fe and unavoidableimpurities, a cold drawing step in which the resulting seamless steeltube is subjected to cold drawing at least one time with a reduction inarea of at least 40% in one time of cold drawing operation to obtain asteel tube having predetermined dimensions, and a heat treatment step inwhich the cold drawn steel tube is subjected to quench hardening byheating it to a temperature of at least the Ac₃ point at a rate oftemperature increase of at least 50° C. per second followed by coolingat a cooling rate of at least 50° C. per second at least in atemperature range of 850-500° C. and then to tempering at a temperatureof at most the Ac₁ point.

Preferred embodiments of a process for manufacturing a steel tube forair bags according to the present invention are as follows.

The steel may optionally further contain one or more of the followingelements:

Mo: less than 0.10%,

at least one of Nb: at most 0.050%, Ti: at most 0.050%, and V: at most0.20%; and

at least one of Ca: at most 0.005% and B: at most 0.0030%.

The contents of Cu, Ni, Cr, and Mo in the steel preferably satisfy thefollowing Equation (1).

Cu+Ni≧(Cr+Mo)²+0.3  (1)

The symbols for elements in Equation (1) indicate the values of thecontent of those elements in mass percent. When Mo is not contained,Mo=0.

The wall thickness of the steel tube after completion of the colddrawing step is preferably at most 2.0 mm.

The cold drawing step is preferably carried out by performing colddrawing a single time.

The heating for quench hardening in the heat treatment step ispreferably carried out by high-frequency induction heating. In thiscase, before being heated for quench hardening, the steel tube obtainedin the cold drawing step preferably undergoes straightening.

According to the present invention, it is possible to manufacture asteel tube for air bags having a high strength as expressed by a tensilestrength of at least 900 MPa and excellent low-temperature toughness asexpressed by vTrs100 of −60° C. or below, while the content of expensiveMo is restricted to 0 or a low level. In addition, the strength of theseamless steel tube obtained by hot tube forming is not too high, so theworking ratio in the subsequent cold drawing step can be increasedcompared to a conventional process, and the number of times that colddrawing operation must be carried out with intervening softening betweencold rolling operations can be decreased. Therefore, according to thepresent invention, it is possible to decrease both the alloy cost andthe manufacturing cost of a steel tube for air bags compared to theprior art.

Modes for Carrying Out the Invention

The chemical composition and the manufacturing process for a steel tubefor air bags according to the present invention will be explained morespecifically below.

(A) Chemical Composition of the Steel

In this description, percent with respect to the chemical composition ofa steel means mass percent. The remainder of the chemical composition ofa steel other than the elements described below is Fe and unavoidableimpurities.

C: 0.04-0.20%

C is an element which is effective at inexpensively increasing thestrength of steel. If its content is less than 0.04%, it is difficult toobtain a high strength (tensile strength), and if it exceeds 0.20%,workability and weldability decrease. Accordingly, the C content is madeat least 0.04% and at most 0.20%. A preferred range for the C content isat least 0.07% to at most 0.20%, and a more preferred range is at least0.12% to at most 0.17%. When it is desired to obtain a tensile strengthof at least 1000 MPa, it is preferable to contain at least 0.06% of C.

Si: 0.10-0.50%

Si is an element which has a deoxidizing action and which also increasesthe strength of steel by increasing its hardenability. With this object,the Si content is made at least 0.10%. However, if its content exceeds0.50%, toughness decreases, so the Si content is made at most 0.50%. Apreferred range for the Si content is at least 0.20% to at most 0.45%.

Mn: 0.10-1.00%

Mn is an element which has a deoxidizing action and which is alsoeffective at increasing the strength and toughness of steel byincreasing its hardenability. If its content is less than 0.10%, asufficient strength and toughness are not obtained. If its contentexceeds 1.00%, coarsening of MnS takes place, the coarse MnS beingelongated at the time of hot rolling, leading to a decrease intoughness. Therefore, the Mn content is made at least 0.10% and at most1.00%. A preferred Mn content is at least 0.30% and at most 0.80%.

P: at most 0.025%

P, which is contained in steel as an impurity, produces a decrease intoughness due to grain boundary segregation. In particular, if the Pcontent exceeds 0.025%, toughness is markedly decreased. Accordingly,the P content is made at most 0.025%. The P content is preferably atmost 0.020% and more preferably at most 0.015%.

S: at most 0.005%

S, which is contained in steel as an impurity, also decreases toughnessparticularly in the T direction of a steel tube (the directionperpendicular to the rolling direction (the lengthwise direction) of asteel tube). If the S content exceeds 0.005%, there is a marked decreasein the toughness in the T direction of a steel tube, so the S content ismade at most 0.005%. A preferred S content is at most 0.003%.

Al: at most 0.10%

Al is an element which has a deoxidizing action and which is effectiveat increasing the toughness and workability of steel. However, if Al iscontained in an amount exceeding 0.10%, there is marked occurrence ofsand marks. Accordingly, the Al content is made at most 0.10%. The Alcontent may be on the level of an impurity, so there is no particularlower limit, but it is preferably at least 0.005%. The Al content in thepresent invention is expressed as the content of acid-soluble Al(so-called sol. Al).

Cr: 0.01-0.50%

Cr has the effect of increasing the strength and toughness of steel byincreasing the hardenability and resistance to temper softening. Thiseffect appears when the Cr content is at least 0.01%. However, becauseCr is an element which improves hardenability, it causes hardening ofsteel in the cooling stage after hot tube forming, thereby limiting theworking ratio in a single time of cold drawing operation, so there is anincreased necessity to perform cold drawing a plurality of times in acold drawing step with intervening softening treatment. Furthermore, anincrease in the Cr content leads to an increase in the alloy cost. Forthe above reasons, the Cr content is made at least 0.01% and at most0.50%. A preferred Cr content is at least 0.15% to at most 0.45%, and amore preferred content is at least 0.18% to at most 0.35%.

Mo: 0% to less than 0.10 mass %

Mo has the effect of increasing the strength and toughness of steel byincreasing the hardenability and resistance to temper softening. Thiseffect appears when its content is at least 0.01%. However, in thepresent invention, the necessary strength and toughness are achieved byNi and Cu, and it is not essential to add Mo. Namely, Mo may be 0%.

When Mo is added, its content is made less than 0.10%. If the Mo contentis higher, even if a seamless steel tube obtained by hot tube forming isair cooled, there is a tendency for the strength of the seamless steeltube to become too high. As a result, in the subsequent cold drawingstep, it becomes necessary to carry out softening before working, andthe working ratio (reduction in area) in cold drawing is limited.Therefore, the number of times of cold drawing and softening prior tocold drawing necessary to obtain a steel tube having predetermineddimensions increases. This tendency becomes marked when Mo is 0.10% orgreater. Mo is an extremely expensive metal, so an increase in the Mocontent is tied to a marked increase in the alloy cost. Namely, an Mocontent of 0.10% or higher is an impediment to achieving the objects ofthe present invention. Accordingly, when Mo is contained, its content ismade less than 0.10%, and a preferred content of Mo is at least 0.01%and at most 0.05%.

Cu: 0.01-0.50%

Cu has the effect of increasing the strength and toughness of steel byincreasing its hardenability. This effect is exhibited if the Cu contentis at least 0.01% and preferably at least 0.03%. However, a Cu contentin excess of 0.50% to leads to an increase in the alloy cost.Accordingly, the Cu content is made at least 0.01% and at most 0.50%. Apreferred Cu content is at least 0.03% and particularly at least 0.05%,and more preferably at least 0.15%. The upper limit on the Cu content ispreferably 0.40% and more preferably 0.35%.

Ni: 0.01-0.50%

Ni has the effect of increasing the strength and toughness of steel byincreasing its hardenability. This effect appears if the Ni content isat least 0.01% and preferably at least 0.03%. However, an Ni contentexceeding 0.50% leads to an increase in the alloy cost. Accordingly, theNi content is made at least 0.01% and at most 0.50%. The Ni content ispreferably at least 0.03%, more preferably at least 0.05%, and mostpreferably at least 0.15%. The upper limit on the Ni content ispreferably 0.40% and more preferably 0.35%.

The sum of the contents of Cu and Ni (Cu+Ni) is preferably at least0.20% and at most 0.65%, and more preferably at least 0.28% and at most0.60%.

In a preferred embodiment of the present invention, the contents of Cu,Ni, Cr, and Mo in steel are adjusted so as to satisfy the followingEquation (1).

Cu+Ni≧(Cr+Mo)²+0.3  (1)

The symbols for elements in Equation (1) indicate the value of thecontent of each element in mass percent. When the steel does not containMo, Mo is 0.

Cr and Mo interfere with spheroidization of cementite which precipitatesduring tempering. Particularly in a steel containing B, they easily formcompounds with B (borides) at grain boundaries, so they easily cause adecrease in toughness particularly in a high-strength steel. Bysuppressing Cr and Mo and containing Cu and Ni so as to satisfy Equation(1), it becomes easy to manufacture a steel tube for air bags having ahigh strength and a high toughness.

In a preferred embodiment of the present invention, at least one elementselected from one or both of the following groups (i) and (ii) can befurther contained.

(i) Nb, Ti, V

(ii) Ca, B

Nb: at most 0.050%

Nb, which is finely dispersed in steel as carbides, has an effect ofstrongly pinning grain boundaries. As a result, it refines crystalgrains and increases the toughness of steel. However, if Nb is containedin an amount exceeding 0.050%, carbides coarsen and toughness ends updecreasing. Accordingly, when Nb is added, its content is made at most0.050%. The above-described effect of Nb appears even with an extremelysmall content, but in order to adequately obtain this effect, the Nbcontent is preferably at least 0.005%.

Ti: at most 0.050%

Ti has the effect of fixing N in steel and thereby increasing toughness.Finely-dispersed Ti nitrides strongly pin grain boundaries and refinecrystal grains, thereby increasing the toughness of steel. However, ifTi is contained in an amount larger than 0.050%, nitrides coarsen andtoughness ends up decreasing. Accordingly, the content of Ti when it isadded is made at most 0.050%. The effect of Ti appears even when it isadded in a minute amount, but in order to adequately obtain its effect,its content is preferably at least 0.005%. A preferred Ti content is0.008-0.035%.

V: at most 0.20%

V has the effect of ensuring toughness and increasing strength byprecipitation strengthening. However, a V content exceeding 0.20% leadsto a decrease in toughness. Accordingly, the content of V when it isadded is made at most 0.20%. The effect of V appears even when it isadded in a minute amount, but in order to obtain an adequate effect, itscontent is preferably at least 0.02%. A preferred range for the Vcontent is 0.03-0.10%.

Ca: at most 0.005%

Ca has the effect of fixing S, which is present in steel as anunavoidable impurity, as sulfides and improving the anisotropy oftoughness, thereby increasing the toughness in the T direction of asteel tube and hence increasing the resistance to bursting thereof.However, if Ca is contained in excess of 0.005%, inclusions increase andtoughness ends up decreasing. Accordingly, the content of Ca when it isadded is made at most 0.005%. The above-described effect of Ca isobserved even when it is added in an extremely small amount, but inorder to obtain an adequate effect, its content is preferably at least0.0005%.

B: at most 0.0030%

When B is added in a minute amount, it segregates at grain boundaries insteel and markedly increases the hardenability of steel. However, if theB content is 0.0030% or higher, coarse borides precipitate at grainboundaries and a tendency for toughness to decrease is observed.Accordingly, when B is added, its content is made at most 0.0030%. Theeffect of B is observed even when it is added in a minute amount, but inorder to guarantee an adequate effect, its content is preferably made atleast 0.0005%.

In the present invention, when it is desired to obtain a tensilestrength of at least 1000 MPa, it is preferable to add B in order toincrease strength by improving hardenability.

B does not segregate at grain boundaries unless it is present in solidsolution in steel. Accordingly, N, which easily forms a compound with B,is preferably fixed by Ti, and B is preferably contained in at least anamount which is fixed by N. For this reason, the B content preferablysatisfies the relationship given by the following Equation (2) based onthe stoichiometric ratios of B, Ti, and N.

B—(N—Ti/3.4)×(10.8/14)≧0.0001  (2)

In Equation (2), B, N, and Ti represent the values of the contents ofthose elements in mass percent.

(B) Tube Forming Step

A steel ingot of a steel having its chemical composition adjusted as setforth above in (A) is used as a starting material to obtain a seamlesssteel tube by hot tube forming.

There are no particular limitations on the form or the method for thepreparation of a steel ingot which is used as a starting material forhot tube forming. For example, it may be a cast member (a round CCbillet) obtained by casting using a continuous casting machine having acylindrical mold, or it may be an ingot which is cast into a rectangularmold and then hot forged to obtain a cylindrical shape. As a result ofsuppressing the addition of ferrite-stabilizing elements such as Cr andMo and adding austenite-stabilizing elements such as Cu and Ni, evenwhen continuous casting is employed into a round shape to form a roundCC billet, the effect of preventing center cracks is sufficientlyobtained, so the applicability of the present invention to a round CC issufficiently high. As a result, it is possible to eliminate a step ofworking to form a round billet by blooming or the like which isnecessary when casting into a rectangular mold.

There are no particular limitations on a hot tube forming method forobtaining a seamless steel tube. For example, the mandrel-Mannesmannmethod can be used. Cooling after hot tube forming is preferably coolingwith a low cooling rate such as air cooling in order to facilitate colddrawing. There are no particular limitations on the shape of theresulting seamless steel tube, but a diameter of 32-50 mm and a wallthickness of around 2.5-3.0 mm, for example, are suitable.

(C) Cold Drawing Step

A seamless steel tube which is obtained by hot tube forming generallyhas a large wall thickness and a large diameter with an inadequatedimensional accuracy. In order to obtain predetermined dimensions (theouter diameter and wall thickness of a steel tube) and good surfacecondition, the seamless steel tube which is used as a mother tube issubjected to cold drawing. In the present invention, in order to exploitthe characteristics of the steel being used, the working ratio(reduction in area) in at least one time of cold drawing operation whichis performed in the cold drawing step is made greater than 40%. If theworking ratio in one time of cold drawing operation exceeds 50%, innersurface wrinkles and cracks easily develop, so the working ratio ispreferably 42-48% and more preferably 43-46%. When cold drawing iscarried out two or more times in the cold drawing step, the workingratio in at least one of the times should be at least 40%, and it ispossible to combine cold drawing having a working ratio of at least 40%with cold drawing having a working ratio of less than 40%.

The working ratio in cold drawing is synonymous with the reduction inarea (decrease in cross section) defined by the following formula.

% reduction in area=(S ₀ −S _(f))×100/S ₀

where, S₀ is the cross-sectional area of the steel tube before colddrawing, and S_(f) is the cross-sectional area of the steel tube afterthe completion of cold drawing.

The cross-sectional area of a steel tube is the cross-sectional area ofjust the tube wall and excludes the hollow portion of the tube crosssection.

The working ratio (or reduction in area) in one time of cold drawingoperation can be the total working ratio when cold drawing is performeda plurality of times with no softening intervening between occurrencesof cold drawing. Using a steel according to the present invention, theworking ratio in one time of cold drawing can exceed 40%, so if thefinished dimensions of a seamless steel tube obtained by hot tubeforming are suitably selected, it is possible to manufacture athin-walled steel tube of predetermined dimensions in a singleoccurrence (one time) of cold drawing. Manufacture can thus be greatlysimplified compared to the conventional process for manufacturing athin-walled steel tube, which requires two occurrences of cold drawingand requires intervening softening between them.

Methods of cold drawing are well known, and cold drawing can be carriedout in a conventional manner. For example, when a seamless steel tubeprepared by the mandrel-Mannesmann method as described above is used asa mother tube, the resulting tube may be allowed to cool to roomtemperature and then subjected to drawing with a die and a plug toreduce the diameter and wall thickness of the tube. A steel tube for airbags preferably has a diameter of at most 30 mm and a wall thickness ofat most 2 mm, for example. As long as a steel tube having the necessarydimensions can be obtained from the seamless steel tube used as a mothertube by cold drawing, there are no particular limitations on the workingmethod, but the above-described drawing method is preferable.

With a steel composition used in the present invention, it is possibleto perform working with a reduction in area of 46%, for example, bysingle occurrence of cold drawing. Therefore, when the final dimensionsof a steel tube for air bags are a wall thickness of 1.7 mm and an outerdiameter of 25 mm, if the dimensions of a mother tube to undergo colddrawing are, for example, an outer diameter of 31.8 mm and a wallthickness of 2.5 mm, it is possible to obtain a product havingpredetermined dimensions by performing cold drawing a single time.

(D) Straightening

Since a steel tube for air bags manufactured in the present inventionhas a tensile strength of at least 900 MPa and has undergone colddrawing with a reduction in area of at least 40%, there is a tendencyfor the strength of the steel tube after cold drawing to be higher thanfor a conventional steel, and in some cases, there is the possibility ofthe steel tube developing bending such as springback after cold drawing.

As explained below, in order to achieve a high strength and hightoughness, a steel tube which is given predetermined dimensions by colddrawing is heated to at least the Ac₃ transformation point by rapidheating for the purpose of quench hardening. This rapid heating istypically carried out by high-frequency induction heating. If there arebends in a steel tube which is to undergo quench hardening, to theproblem may occur that the steel tube is unable to pass straight throughthe high-frequency coils used for high-frequency induction heating.Accordingly, in a preferred embodiment, straightening is carried outafter cold drawing to remove bends in the steel tube.

There are no particular limitations on the straightening method, and isstraightening can be carried out in a conventional manner. For example,a preferred method is one in which four 2-roll stands having an adjustedroll gap are provided with the center of the roll gap in each standbeing slightly deviated or offset with respect to each other and a steeltube is passed through the rolls to apply working in the form of bendingforth and back. The higher the working ratio in bending forth and backat this time, the higher is the effect of straightening. From thisstandpoint, the amount of offset (the amount of deviation of the rollaxis between adjacent roll pairs) is made at least 1% of the outerdiameter of the steel tube, and the roll gap is preferably made at most1% smaller than the outer diameter of the steel tube. In order to avoidproblems such as cracking of the steel tube, the amount of offset ispreferably made at most 50% of the outer diameter of the steel tube, andthe roll gap is preferably made at least 5% smaller than the outerdiameter of the steel tube.

(E) Heat Treatment

After carrying out the straightening described above in (D) as required,the steel tube is subjected to heat treatment in order to impart therequired tensile strength to the steel tube and increase the toughnessin the T direction, thereby guaranteeing bursting resistance. In orderto give a steel tube a high strength as expressed by a tensile strengthof at least 900 MPa and excellent low temperature toughness or burstingresistance, heat treatment is carried out by quench hardening afterheating to a temperature of at least the Ac₃ (transformation) point andsubsequent tempering at a temperature of at most the Ac₁(transformation) point.

If the heating temperature before quenching is lower than the Ac₃ pointat which an austenite single phase forms, it is not possible toguarantee good toughness in the T direction (and accordingly goodbursting resistance). On the other hand, if the heating temperature istoo high, austenite grains abruptly start to grow and become coarsegrains, and toughness decreases. Therefore, the heating temperature ispreferably made at most 1050° C. More preferably it is at most 1000° C.

Heating to at least the Ac₃ point for quench hardening is carried out byrapid heating at a heating rate of at least 50° C. per second. Thisheating rate can be the average heating rate in a temperature range fromat least 200° C. to the heating temperature. If the heating rate islower than 50° C. per second, it is not possible to achieve refinementof austenite grain diameters, and the tensile strength andlow-temperature toughness or bursting resistance decrease. In order toobtain a steel tube with a tensile strength of at least 1000 MPa andvTrs100 of −80° C. or below, the heating rate is preferably at least 80°C. per second and more preferably at least 100° C. per second. Thisrapid heating can be achieved by high-frequency induction heating. Inthis case, the heating rate can be adjusted by the feed speed of a steeltube passing through high-frequency coils.

A steel tube which has been heated to a temperature of at least the Ac₃point by rapid heating is held for a short period at a temperature of atleast the Ac₃ point, and then it is rapidly cooled to carry out quenchhardening. The holding time is preferably in the range of 0.5-8 seconds.More preferably it is 1-4 seconds. If the holding time is too short, theuniformity of mechanical properties is sometimes inferior. If theholding time is too long, particularly if the holding temperature is onthe high side, it easily leads to coarsening of the austenite graindiameter. Refinement of grain diameter is necessary to guaranteeextremely high toughness.

The cooling rate for quench hardening is controlled so as to be at least50° C. per second at least in a temperature range of 850-500° C. Thiscooling rate is preferably at least 100° C. per second. In order to makethe tensile strength at least 1000 MPa and make vTrs100 a value of −80°C. or below, the cooling rate is preferably made at least 150° C. persecond. If the cooling rate is too low, quench hardening becomesincomplete, and the proportion of martensite decreases, so a sufficienttensile strength is not obtained.

A steel tube which has undergone the above-described rapid cooling andcooled to the vicinity of room temperature is then subjected totempering at a temperature of at most the Ac_(t) point in order toimpart a tensile strength of at least 900 MPa and sufficient burstingresistance. If the tempering temperature exceeds the Ac₁ point, itbecomes difficult to stably obtain the desired tensile strength andlow-temperature toughness with certainty.

There are no particular limitations on a method for tempering, and itcan be carried out by, for example, soaking in a heat treatment furnacesuch as a hearth roller type continuous furnace or by usinghigh-frequency induction heating or the like followed by cooling.Preferred soaking conditions in a heat treatment furnace are atemperature of 350-500° C. and a holding time of 20-30 minutes. Aftertempering, bends can be straightened using a suitable straightener orthe like in the manner described in (D).

In order to form an air bag accumulator from a steel tube for air bagsmanufactured in this manner, the steel tube is cut to a predeterminedlength to obtain a short tube, and if necessary at least one end of thecut tube is subjected to diameter reduction by press working or spinning(this is referred to as bottling) for final working to a shape necessaryfor mounting of an initiator or the like. Accordingly, the predetermineddimensions and dimensional accuracy for a steel tube for air bagsreferred to in this description mean the dimensions and dimensionalaccuracy with respect to the tube thickness and diameter. Finally, a lidis mounted on each end of the steel tube by welding.

EXAMPLES

Steels having the chemical compositions shown in Table 1 with Ac₁ pointsin the range of 720-735° C. and Ac₃ points in the range of 835-860° C.were prepared in a converter, and cylindrical billets having an outerdiameter of 191 mm were manufactured by continuous casting (round CC).Each round CC billet was cut to a desired length and heated to 1250° C.,and then it underwent piercing and rolling by the usual Mannesmannpiercer-mandrel mill type technique to obtain a first mother tube havingan outer diameter of 31.8 mm and a wall thickness of 2.5 mm and a secondmother tube having an outer diameter of 42.7 mm and a wall thickness of2.7 mm.

The two types of mother tubes which were obtained in this mannerunderwent cold drawing one or two times by a usual method which carriesout drawing using a die and a plug and were finished to form steel tubeswith an outer diameter of 25.0 mm and a wall thickness of 1.7 mm. Forcomparative steels G and H in Table 1, when it was attempted tomanufacture a steel tube having the above-described shape by performingcold drawing one time on the first mother tube having an outer diameterof 31.8 mm and a wall thickness of 2.5 mm, fracture developed andmanufacture could not be carried out.

In Comparative Examples 9 and 10, the second mother tubes were used. Asteel tube having an outer diameter of 32.0 mm and a wall thickness of2.2 mm was formed by performing drawing a first time, then it underwentsoftening at 630° C. for 20 minutes, and then it was finished to anouter diameter of 25.0 mm and a wall thickness of 1.7 mm by performingdrawing a second time.

Each steel tube which underwent cold drawing was straightened using astraightener, and then it was subjected to water quenching by heating to920° C. at an average heating rate of 300° C. per second (the averagevalue in the temperature range of 200-900° C.) using a high-frequencyinduction heating apparatus, holding at 920° C. for 2 seconds, and watercooling (at an average cooling rate of 150° C. per second in thetemperature range of 850-500° C.). Subsequently, in order to temper thesteel tube, it was soaked for 30 minutes at 350-500° C. in a brightannealing furnace and then cooled to room temperature by natural coolinginitially in the furnace and then outside the furnace to obtain a steeltube for air bags.

A tube of a fixed length was cut from each resulting steel tube, and itwas cut in the lengthwise direction of the tube at room temperature andunrolled. A rectangular member having a length of 55 mm, a height of 10mm, and a width of 1.7 mm which was taken in the T direction from theunrolled tube and which had a 2-mm V-notch was used as a test piece fora Charpy impact test which was carried out at various temperatures of−40° C. and below. By means of this test, the lowest temperature atwhich the percent ductile fracture was 100% (vTrs100) was obtained.

Using a No. 11 test piece prescribed by JIS Z 2201 which was taken fromthe L direction of each steel tube, a tensile test in accordance withthe tensile test method for metals prescribed by JIS Z 2241 was carriedout. The results of the above tests and the manufacturing conditions ofa steel tube are compiled in Table 2.

TABLE 1 (Cr + Cu Mo)² Steel composition (mass %, remainder of Fe andimpurities) + + Re- Steel C Si Mn P S Cr Mo Cu Ni Nb Ti V sol. Al Ca BNi 0.3 mark A 0.14 0.29 0.50  0.012 0.003 0.30  0.01  0.25 0.26 0.0250.024 — 0.031 0.0016 0.0014 0.51 0.40 This B 0.15 0.28 0.48  0.012 0.0020.29  — 0.26 0.28 0.024 0.024 — 0.035 0.0011 0.0013 0.54 0.38 in- C 0.140.26 0.52  0.013 0.002 0.30  0.01  0.27 0.25 0.024 0.026 — 0.042 0.00150.0014 0.52 0.40 ven- D 0.13 0.25 0.47  0.011 0.002 0.36  0.04  0.260.06 — 0.023 0.018 0.042 0.0013 0.0015 0.32 0.46 tion E 0.13 0.26 0.48 0.012 0.002 0.22  — 0.26 0.25 — — — 0.034 — — 0.51 0.35 F 0.15 0.260.40  0.013 0.003 0.35  0.02  0.29 0.30 — 0.022 — 0.040 — 0.0010 0.590.44 G 0.12 0.25 1.29* 0.014 0.003 0.61* 0.28* 0.27 0.25 0.023 0.024 —0.036 0.0015 0.0003 0.52 1.09 Comp- H 0.15 0.23 0.54  0.013 0.002 0.74*0.35* 0.29 0.31 0.024 0.008 — 0.033 0.0022 0.0002 0.60 1.49 para- tive*Outside the range defined herein.

TABLE 2 Total First cold rolling Second cold rolling work- Heating Cool-Dimensions % area % area ing conditions ing Run of mother Dimensionsreduc- Re- Dimensions reduc- Re- ratio for quench rate TS vTrs100 No.Steel tube (mm) tion sult (mm) tion sult (%) hardening (° C./s) (MPa) (°C.) Remark  1 A OD: 31.8 mm × OD: 25.0 mm × 46   ∘ — — — 46   920° C. ×2s 150 1098 −120 This  2 B 2.5 mm t 1.7 mm t ∘ — — — (high 1070 −120inven-  3 C ∘ — — — frequency 1101 −120 tion  4 D ∘ — — — induction 1022 −75  5 E ∘ — — — heating) 1028 −100  6 F ∘ — — — 1053 −110  7 G x ****** *** *** *** *** *** Com-  8 H x *** *** *** *** *** *** *** para-  9G OD: 42.7 mm × OD: 32.0 mm × 39.3 ∘ OD: 25.0 mm × 39.6 ∘ 63.3 920° C. ×2s 150 1075 −110 tive 10 H 2.7 mm t 2.2 mm t ∘ 1.7 mm t ∘ (HF-IH) 1040−110 *** Due to cracking which occurred during cold drawing, subsequentsteps could not be preformed. HF-IH = high frequency induciton heating

As is apparent from Table 2, when steels A-F having the chemicalcomposition of a steel according to the present invention were used, inspite of a low alloy cost due to the amount of expensive Mo which waszero or a small amount of less than 0.10%, it was possible to performworking to predetermined product dimensions by one time of cold drawingeven with a working ratio as expressed by a reduction in area of 46%.Furthermore, by carrying out rapid heating and rapid cooling in thesubsequent quench hardening step, it was possible to achieve a highlevel of product performance as a steel tube for air bags. Inparticular, when using steels A-C, E, and F having a composition whichsatisfies above-described Equation (1), vTrs100 was −100° C. or below,so it is apparent that the low-temperature toughness is extremely highand excellent bursting resistance in a low-temperature environment canbe expected.

Steels F and G, which were comparative examples, contained a largeamount of Mo, so the alloy cost was high. Furthermore, cracks developedwhen cold drawing was carried out with a reduction in area of at least40%. Therefore, it is necessary to carry out cold drawing at least 2times with a reduction in area of less than 40%, and softening betweencold drawing is necessary, so the manufacturing costs of a steel tubefor air bags also increase.

1. A process for manufacturing a steel tube for air bags characterizedby including: a tube forming step in which a seamless steel tube isproduced by hot tube forming from a steel comprising, in mass %, C:0.04-0.20%, Si: 0.10-0.50%, Mn: 0.10-1.00%, P: at most 0.025%, S: atmost 0.005%, Al: at most 0.10%, Cr: 0.01-0.50%, Cu: 0.01-0.50%, Ni:0.01-0.50%, and a remainder of Fe and unavoidable impurities, a colddrawing step in which the resulting seamless steel tube is subjected tocold drawing at least one time with a reduction in area of at least 40%in one time of cold drawing to obtain a steel tube having predetermineddimensions, and a heat treatment step in which the cold drawn steel tubeis subjected to quench hardening by heating it to a temperature of atleast the Ac₃ point at a rate of temperature increase of at least 50° C.per second followed by cooling at a cooling rate of at least 50° C. persecond at least in a temperature range of 850-500° C. and then totempering at a temperature of at most the Ac₁ point.
 2. A process formanufacturing a steel tube for air bags as set forth in claim 1 whereinthe steel further contains less than 0.10% of Mo.
 3. A process formanufacturing a steel tube for air bags as set forth in claim 1 whereinthe steel contains at least one of Nb: at most 0.050%, Ti: at most0.050%, and V: at most 0.20%.
 4. A process for manufacturing a steeltube for air bags as set forth in claim 1 wherein the steel contains atleast one of Ca: at most 0.005% and B: at most 0.0030%.
 5. A process formanufacturing a steel tube for air bags as set forth in claim 1 whereinthe contents of Cu, Ni, Cr and Mo in the steel satisfy the followingEquation (1):Cu+Ni≧(Cr+Mo)²+0.3  (1) wherein the symbols for elements in Equation (1)mean the values of the content of the respective elements in masspercent, and Mo=0 when the steel does not contain Mo.
 6. A process formanufacturing a steel tube for air bags as set forth in claim 1 whereinthe wall thickness of the steel tube after completion of the colddrawing step is at most 2.0 mm.
 7. A process for manufacturing a steeltube for air bags as set forth in claim 6 wherein the cold drawing stepis carried out by performing cold drawing one time.
 8. A process formanufacturing a steel tube for air bags as set forth in claim 1 whereinheating for quench hardening in the heat treatment step is carried outby high-frequency induction heating.
 9. A process for manufacturing asteel tube for air bags as set forth in claim 8 wherein the steel tubeobtained in the cold drawing step is straightened before heating for thequench hardening.